In this study we present a measuring method and extensive experimental data on the gas phase RTD in a mechanically agitated gas-liquid reactor with standard dimensions over a wide range of superficial gas velocities, agitation rates and agitator sizes. The results are modelled successfully, using the weighed moments method, by a simple RTD model of one mixer in series with a plug flow zone. All results are correlated in one relation, which can be used for scale-up.